The present invention relates to a variable capacitor and a manufacturing method thereof, and more particularly relates to a variable capacitor using MEMS (Micro Electro Mechanical System) techniques and a manufacturing method thereof.
A variable capacitor is an important component of electric circuits including a variable frequency oscillator, a tuned amplifier, a phase shifter, an impedance matching circuit, etc., and in recent years, an increasing number of portable apparatuses incorporate a variable capacitor. Compared to varactor diodes which are mainly used nowadays, a variable capacitor manufactured using MEMS techniques has the advantage that it can increase the Q value with a small loss, and therefore there is an urgent need for the development of the variable capacitor (see, for example, Japanese Patent Applications Laid-Open No. 2003-188049 and No. 09-162074).
In general, a variable capacitor comprises two facing electrodes, one or both of which is a movable electrode, and changes the capacitance by changing the distance between the two facing electrodes by driving the movable electrode with an actuator.
FIG. 1 is a structural diagram of a conventional typical variable capacitor. A fixed electrode 41 is mounted on a substrate 44, and a movable electrode 42 is resiliently supported by a substrate 44 to face the fixed electrode 41. In FIG. 1, a spring 51 is illustrated functionally to show this resilient support. By adjusting a voltage to be applied between the fixed electrode 41 and movable electrode 42, the balance between the electrostatic attractive force generated between the two electrodes 41, 42 and the strength of the spring 51 is controlled, and an arbitrary distance d is secured between the two electrodes 41 and 42. Further, by changing the distance d, a desired electrostatic capacitance is obtained.
The conventional variable capacitor shown in FIG. 1 has the following problems. When the distance d between the two electrodes 41 and 42 is decreased to ⅔ of the initial value or less, a pull-in phenomenon occurs and the movable electrode 42 is suddenly attracted and comes into contact with the fixed electrode 41, and consequently a short-circuit occurs between the two electrodes 41 and 42. For this reason, in a variable capacitor having such a structure, since the distance d between the two electrodes 41 and 42 can be decreased up to ⅔ of the initial value, the capacitance can be changed only up to 1.5 times the initial value.